The present invention relates to a light conducting plate used in an edge-light type backlighting device for illuminating a transmission-type or semi-transmission type panel from its back side.
In recent years, thin and legible liquid crystal display devices having a backlighting mechanism are used as display devices for lap-top or book-type word processors, computers, etc. These backlighting devices are of an edge-light type in which, as shown in FIG. 1, a rod-shaped light source (4 in the figure) such as a fluorescent tube is disposed adjacent to one end of a transparent light conducting plate (1 in the figure). In many of the edge-light-type backlighting devices, as shown in FIG. 2, one major surface (back surface) of the light conducting plate is partially covered with a light diffuse-reflecting substance shaped in dots and stripes or formed with a number of protrusions or recesses, and that surface is covered with a light diffuse-reflecting sheet (3 in the figure) almost completely- Further, the light exit surface of the light conducting plate is covered with a light diffusing sheet (2 in the figure).
Recently, in particular, the backlighting devices, which are driven by a battery, are desired to have a further improved consumed-power-to-luminance conversion efficiency. To this end, there have been proposed various methods for causing light rays emitted from the light source to efficiently enter the end face of the light conducting plate by making a reflector that encloses the linear light source have a parabolic or elliptical sectional shape, or a special sectional shape (as disclosed in Japanese Unexamined Patent Publication No. Hei. 4-257823) taken perpendicularly to its longitudinal direction.
However, although each of the above methods can improve the consumed-power-to-luminance conversion efficiency, the degree of improvement is still not enough and further improvement is desired.